A chromium catalyst for the polymerization of ethylene as a homogeneous model for the Phillips catalyst

Article abstract of DOI:10.1021/ja043877x

A structurally characterized cationic chromium(III) alkyl featuring a bulky nacnac ligand catalyzes the polymerization of ethylene as well as the copolymerization of ethylene with α-olefins. This well-characterized homogeneous catalyst constitutes a structural as well as functional model of the widely used heterogeneous Phillips olefin polymerization catalyst. Copyright

Full text of DOI:10.1021/ja043877x

Published on Web 01/07/2005
A Chromium Catalyst for the Polymerization of Ethylene as a Homogeneous
Model for the Phillips Catalyst
Leonard A. MacAdams, Gerald P. Buffone, Christopher D. Incarvito, Arnold L. Rheingold, and
Klaus H. Theopold*
Center for Catalytic Science and Technology, Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19716
Received October 7, 2004; E-mail: theopold@udel.edu
In pursuit of a homogeneous model system for the heterogeneous
Phillips catalyst for olefin polymerization,1 we have turned to
chromium coordinated by “nacnac” ligands ((Ar)2nacnac ) 2,4-
pentane-N,N′-bis(aryl)ketiminato).2 The bidentate coordination with
hard N-donors approximates the proposed binding of chromium to
the silica surface via two oxygen linkages, while the N-substituents
allow for steric protection of the active site.3 While our initial
experiments using the (Ph)2nacnacCr(III) fragment demonstrated
catalytic activity,2a subsequent attempts to isolate a well-defined
single-site catalyst were thwarted by disproportionation and ortho-
metalation reactions.2b Herein we describe the structural charac-
terization and reactivity of a cationic chromium alkyl catalyst that
is active in the absence of any cocatalyst.
Addition of (2,6-Me2Ph)2nacnacLi to a slurry of CrCl3(THF)3
in THF yielded the dichloride (2,6-Me2Ph)2nacnacCrCl2(THF)2 (1).
Reaction of 1 with lithium alkyls in ether produced the neu-
Figure 1. Molecular structure of 6 (the BARF anion has been omitted for
tral dialkys (2,6-Me2Ph)2nacnacCr(THF)Me2 (2), (2,6-Me2Ph)2-
clarity). Selected bond distances [Å] and angles [°]: Cr(1)-C(22), 1.930(3);
nacnacCr(CH2SiMe3)2 (3, see Scheme 1), and (2,6-Me2Ph)2nacnacCr-
(Bn)2 (4), all of which have been structurally characterized. 2 adopts
Cr(1)-O(1), 2.056(2); Cr(1)-N(1), 1.934(3); Cr(1)-N(2), 1.930(3);
N(1)-Cr(1)-N(2), 89.77(11); O(1)-Cr(1)-C(22), 95.55(13).
Scheme 1. Synthesis of (2,6-Me2Ph)2nacnacCr(III) Alkyls
a square pyramidal geometry, while 3 and 4 are rare examples of
pseudotetrahedral Cr(III) complexes.
Despite their coordinative and electronic unsaturation-four-
coordinate 3 has an 11-electron configuration-none of the neutral
dialkyls reacted with ethylene. This is in notable contrast to Cp*Cr-
(CH2SiMe3)2, an analogue of 3 that polymerizes ethylene at low
temperatures and pressures.4 Even formation of a cationic nacnac
chromium alkyl does not guarantee catalytic activity. Thus, square
pyramidal [(2,6-Me2Ph)2nacnacCr(THF)2Me]BPh4 (5), which was
prepared by protonolysis of 2 with 1 equiv of HNEt3BPh4, did not
exhibit any polymerization activity. Once again, we note the contrast
to related [Cp*Cr(THF)2Me]BPh4, a functional homogeneous model
for the Union Carbide catalyst.5
However, treatment of 3 with H(OEt2)2+BARF- (BARF-
)
B(3,5-(CF3)2C6H3)4-) in dichloromethane resulted in loss of 1 equiv
of SiMe4 and concomitant formation of the brown ether adduct
[(2,6-Me2Ph)2nacnacCr(OEt2)CH2SiMe3]BARF (6), which could be
recrystallized from CH2Cl2/pentane. The molecular structure of 6
has been determined by X-ray diffraction, and the result is depicted
in Figure 1.6
6 adopted distorted tetrahedral coordination of chromium, with
the N(1)-Cr(1)-O(1) angle of 136° representing the largest
deviation from the ideal tetrahedral angle. The metal-ligand bond
distances are rather short, befitting the cationic nature and low
1
coordination number of 6. The H NMR spectrum of 6 exhibited
isotropically shifted and broadened resonances, as expected for a
paramagnetic chromium(III) species, and the effective magnetic
moment of µeff (295 K) ) 4.1(1) µB is consistent with three unpaired
electrons of a Cr(III) (d3) ion in the inevitable high-spin configu-
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J. AM. CHEM. SOC. 2005, 127, 1082-1083
10.1021/ja043877x CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Me2Ph)2nacnac Cr(THF)CH2SiMe3]BARF. Furthermore, addition
of small amounts of Et2O to catalyst solutions significantly
decreases their activity. Finally, dilution of the catalyst increased
its specific activity; i.e., the effect expected due to a concentration-
dependent shift of a dissociation equilibrium.
Three-coordinate cationic Cr(III) alkyls supported by bidentate
nitrogen ligands catalyze the polymerization of ethylene and the
copolymerization of ethylene with R-olefins. Therefore, we suggest
that a species akin to A represents a reasonable candidate for the
active surface species of the heterogeneous Phillips catalyst. Further
studies concerning the effect of charge, formal oxidation state, and
ligand structure on the reactivity of chromium alkyls with olefins
will be the subject of future publications.
Acknowledgment. This research was supported by grants from
the NSF (CHE-0132017) and Chevron Phillips Chemical Co.
Supporting Information Available: Experimental and character-
ization details for 1-6 (PDF); X-ray structural data for 6 (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Plot of Mn of various polyethylene samples vs ethylene
conversion (0.036 mmol 6, pethylene ) 10-40 psig). Numbers denote
Mw/Mn for each sample.
References
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